The optical response of benign and malignant breast biopsies was recorded using a double beam, ratiorecording UV_VIS_NIR spectrophotometer equipped with a 150 mm diameter integrating sphere. Differences in the oxygenation state of hemoglobin for benign and malignant breast biopsies were found. Specifically, spectral component analysis performed in the 300 - 800 nm wavelength region showed that the malignant samples contained predominantly deoxygenated blood while the benign samples exhibited oxyhemoglobin resonances exclusively. Fibroadenoma, a benign tumor, was found to exhibit a deoxyhemoglobin signature and was thus indistinguishable from cancer. Fibroadenoma is also a false positive for X-ray mammography and contrastenhanced MRI. The in vitro experiments were repeated in vivo using a remote reflectance spectrophotometer and human breast cancer xenografts implanted in nude (immune-deficient) mice. The in vivo studies confirm that (1) a potentially exploitable difference in the functional utilization of oxyhemoglobin exists between benign and malignant breast processes, and (2) the sample collection and in vitro preparation protocols yield tissue samples that adequately  represent (spatial) average functional behavior in vivo. It is postulated that these differences may serve as the basis for the non invasive, differential diagnosis of benign and malignant breast processes, or, for tissue characterization in a minimally invasive environment.